The chemostat is being used for studying the effect of changes in growth conditions on the surface components and extracellular products, particularly lipoteichoic acid, of cariogenic bacteria. The organisms being studied are representative strains of streptococci, lactobacilli and actinomycetes and the changes are being followed by chemical, serological and microscopic procedures. Cell surface carbohydrate polymers are of importance in the identification of bacteria and probably also in their interaction with the environment; extracellular lipoteichoic acid can have important biological effects. The bacterial cell surface is particularly sensitive to changes in growth conditions. The chemostat enables growth conditions to be precisely defined so that this study should lead to a clearer understanding of the effect of the environment on cariogenic bacteria -an important factor in understanding their role in dental caries, in defining methods for their identification, and in developing caries vaccines.